People with their natural lens (crystalline lens) of the eye opacified as a result of cataractogenesis require surgical removal of the diseased lens. This condition, known as aphakia, is incompatible with normal vision due to gross anomalies of the refraction and accommodation caused by the absence of the lens in the dioptric system of the eye, and must be corrected. Correction may be achieved by spectacle or contact high plus lenses, by surgical insertion of an artificial plastic lens in the eye as a substitute for the removed crystalline lens, and by surgical modification of the cornea (keratoplasty and epikeratoplasty).
The spectacle corrective lenses, known as cataract glasses, cannot correct monocular aphakia and have also many other drawbacks (loss of visual field with induced ring scotoma, spherical aberration, distortion, magnifying effect, they are cosmetically undesirable). Correction of aphakia through corneal surgery is still limited to investigative conditions. The remaining two procedures, contact lens and artificial intraocular lens, have become presently the best ways for rehabilitation of the cataract patients. It is to be noted that the main use of contact lenses is for correction of vision in the phakic persons wherefore they are thinner than those intended as corrective lenses for the aphakic persons. Both contact and intraocular lenses are currently manufactured from polymeric materials, either hard, generally poly(methyl methacrylate), or soft, when the materials are water-swellable hydrogels of an acrylic or non-acrylic nature, or various hydrophobic flexible polymers. This invention relates to soft hydrophilic materials for the manufacture of both corrective contact lenses for the aphakic patients, and artificial intraocular lenses.
A significant portion of the non-ionizing electromagnetic radiation emanating from the sun, which encompasses ultraviolet (100 to 400 nanometers wavelength), visible (400 to 780 nanometers) and infrared (780 nanometers to 1 millimeter) regions, is potentially harmful to the structural components of the eye, especially to the retina, through thermal and photochemical processes. Except for the cornea which is exposed to the whole atmospheric radiation, that is only from 285 nanometers onwards since the radiation below this wavelength is absorbed by the ozone layer, the other segments of the eye are progressively and selectively protected by the absorbing action of the preceding tissues. The eye therefore appears not only as a complex hydrodynamic system which assures the stability and regulation of the intraocular pressure, and not only as a perfect dioptric system producing a detailed image on the retina, but also as a proper filtering system consisting of a consecutive series of filters which ultimately protect the retina against the harmful effects of certain radiation wavelengths. As a result the human retina in its adult stage is exposed exclusively to radiation wavelengths between 400 and 1400 nanometers, since the remaining incident radiation is absorbed by the cornea, aqueous humor, crystalline lens and vitreous body.
The natural lens is an essential component of the filtering system. From age twenty on, the crystalline lens absorbs most of the ultraviolet radiation between 300 and 400 nanometers, a region known as UV-A. Absorption is enhanced and shifted to longer wavelengths as the lens grows older and it expands eventually over the whole visible region. This phenomenon is correlated with the natural production of fluorescent chromophores in the lens and their age-dependent increasing concentration. Concomitantly, the lens turns yellower due to generation of certain pigments by the continuous photodegradation of the molecules which absorb in the UV-A region. This progressive pigmentation is responsible for the linear decrease in transmission of visible light, since the almost complete absorption in the UV-A region remains constant after age twenty-five.
The damaging effects of intense natural light to the retina, especially of the long-wavelength ultraviolet radiation (UV-A, 300 to 400 nanometers) and short-wavelength visible radiation (400 to 510 nanometers) were noticed some time ago. The acute ultraviolet hazards apply when the eye is exposed to excessive amounts of radiation. These hazards, occurring commonly in certain industrial environments, are well recognized and prevented by the use of regulated or standardized protection equipment. Similarly, the eye is protected from acute injury of the visible radiation by involuntary aversion reflexes of the eye itself, as blinking. However, more subtle photochemical effects induced by the daily exposure to relatively low levels of UV-A radiation and visible radiation at the violet/blue end of the spectrum have been appreciated recently and they are of a greater concern. The retina is very vulnerable to UV-A radiation and the damage inflicted is extensive, as demonstrated on experimental animals. The sensitivity of the retina to short-wavelength visible radiation, known as `blue light hazard region`, is lower but this radiation is ubiquitous and reaches the eye unhampered during the entire diurnal life. Both UV-A radiation and blue light are linked with the age-related degeneration of the retina, as described in: Kirkness, C. M. and Weale, R. A., Transactions of the Ophthalmological Societies of the United Kingdom, vol. 104, pp. 699-702 (1985), "Does Light Pose a Hazard to the Macula in Aphakia ?"; Marshall, J., Ophthalmic and Physiological Optics, vol. 5, pp. 241-263 (1985), "Radiation and the ageing eye"; Mainster, M. A., Eye, vol. 1, pp. 304-310 (1987), "Light and Macular Degeneration: A Biophysical and Clinical Perspective"; and Young, R. W., Survey of Ophthalmology, vol. 32, pp. 252-269 (1988), "Solar Radiation and Age-related Macular Degeneration". The experimental evidence, at least for the blue light hazard, is compelling, and the specialists recommend adequate protection by filtering off as much as possible radiation from 300 to 510 nanometers. This is precisely the work performed by the adult natural lens as part of the filtering system of the eye. In the aphakic eye the most important filter in this system is therefore lost and the age-compromised retina is suddenly exposed to a large dose of harmful radiation.
It is clear that any artificial ocular device intended to act as a substitute for the natural lens must duplicate its filtering properties. Therefore, the materials used to manufacture corrective contact lenses and intraocular lenses should possess adsorptive characteristics matching those of the natural lens.
It is the prime object of this invention to provide a method for obtaining hydrophilic polymers capable of absorbing visible radiation to the same extent as the natural lens. When ultraviolet absorbing agents known in the art are incorporated, the procedure will allow for hydrophilic polymers to be produced which mimic the radiation-absorptive, hence photoprotective properties of the human natural lens. This invention further relates to ocular devices for correction of aphakia, particularly soft intraocular lenses and soft contact lenses, which are made from these polymers.